Plasma aftertreatment of exhaust gases has previously been identified as a possible remediation technique since non-thermal plasma can induce a host of new chemical reactions due to electron excitation thereby causing the production of an abundant amount of radicals and excited state molecules. Plasma aftertreatments rely upon the generation of high local electric fields which directly produce energetic electrons. These energetic electrons can influence the chemistry, even the collision dominated regime, because they do not lose much energy in elastic collision due to their small mass, but instead bounce around and transfer most of their energy to molecules; either dissociating, ionizing, or otherwise exciting them. This excitation and radical production can cause extensive changes in reaction rates; and in some cases, by as much as a hundred thousand-fold increase.
While the efficiency of non-thermal plasma may be limited by any of several factors; two limitations that especially inhibit efficient utilization of non-thermal plasma discharge in connection with aftertreatment of exhaust gases are low electrical efficiency and an undesirable reaction pathway. See B. M. Penetrante et al., "Comparison of Electrical Discharge Techniques for Nonthermal Plasma Processing of NO in N2," pp. 679-687 in IEEE Transactions in Plasma Science, Vol. 23, No. 4, 1995, the relevant portions of which are incorporated herein by reference.
In the excited state chemistry of non-thermal plasma, it is desirable to produce as high an electric field as possible. This would at first seem to simply entail applying high enough voltages to a suitably arranged configuration of electrodes. However, for non-thermal plasma densities, there occurs considerable plasma shielding of the applied fields, even in the collision dominated regime and field limits. See J. H. Whealton and R. L. Graves, "Exhaust Remediation Using Non-Thermal (Plasma) Aftertreatments: A Review," Proceedings of the 1995 Diesel Engine Emissions Reductions Workshop, Vol. 23, LaJolla, calif., the relevant portions of which are incorporated herein by reference. This space charge shielding is due to the charge imbalances arising within the plasma because of the higher mobility of electrons as compared to positive ions. Accordingly, the electric fields are generally limited by high voltage breakdowns which in turn leads to low impedance discharge. Further, plasma shielding effects take place for fields at frequencies below the electron plasma frequency. As a result of this shielding, the volume of plasma where the more favorable high field enhanced reduction reactions occur is reduced to either within a few Debye lengths or an electron energy relaxation distance of the plasma edge thereby diminishing a high field reaction volume by orders of magnitude.
As noted above, problems also arise with respect to the chemistry path. While both oxidation and reduction reaction pathways are possible avenues for the dissociation of NO.sub.x, their respective chemistries differ. The oxidation reaction pathway will result in the production of compounds that include N.sub.2 O and nitric acid. Since nitric acid is toxic, it's generation is to be avoided, especially in automobiles, trucks or other mobile applications. On the other hand, the dissociative attachment occurring in the reduction reaction pathway is more favorable since it lead to the formation of benign N.sub.2. Unfortunately, the above noted difficulty in reaching high E/N in prior art discharges has prevented development of a plasma aftertreatment device that will achieve a high fraction reduction of NO.sub.x as opposed to the less desirable oxidation.
In view of the above problems and deficiencies of plasma aftertreatments of exhaust gases, the present invention was developed.